1. Field of the Invention
The present invention is directed to a method and apparatus for inspecting a transparent container, having a defect on the container bottom, and more particularly to a method and apparatus for such inspection in which a zone of inspection is optically determined in exact coincidence with the container bottom for reliable inspection free from a background error.
2. Description of the Prior Art
It is known in the art to utilize optical scanning technique for detection of a defect or contamination on the bottom of a transparent container such as a beer bottle, in which a diffused illumination is directed through the container bottom to monitor and analyze a variation in the light energy transmitted through the container bottom and detect a defect or contamination when a critical variation in the light energy is found.
The above inspection technique has a problem when utilized in a feed line, i.e., washing line or the like container processing line for successively inspecting the containers moving at a high speed along the line. The problem encountered is the difficulty in obtaining an exact registration of the monitored field of view with the actual container bottom ever moving through an inspection station. Such misregistration will eventually lead to an erroneous inspection result since the system cannot recognize the exact image of the container bottom rather images in processed with regard to an unintended portion outside of the container bottom. For example, when such misregistration occurs, the system may recognize the image of the portion outside of the container bottom as indicative of a defect within the container bottom, resulting in a false detection. This problem becomes more and more critical, particularly when the inspection is required for the containers moving fast the processing line.
Consequently, there is an serious need to define an exact inspection zone in correspondence to the actual container bottom so that the optical analysis can be correctly limited to optical information only within the actual container bottom and without being confused by erroneous information coming from the outside of the container bottom. However, with the known optical inspection systems, such as disclosed in Japanese Patent Publication (KOKAI) No. 57-12352, it is difficult or substantially impossible to define an inspection zone in exact coincidence with the actual container bottom. The reason can be discussed with reference to FIGS. 15 and 16 which schematically illustrate the conventional container bottom inspection system in which diffused illumination L is directed through the bottom of a container 1 to be monitoring by a camera 40 positioned upwardly of the container 1. In the conventional system, the diffused illumination is selected to be uniform in intensity across the container bottom. Thus, as illustrated in FIG. 15A, the light directed to the peripheral edge of the container bottom is considerably deflected thereat and also the light directed to the sidewall of the container is reflected thereat because of its small angle of incidence, so that there is seen a considerable reduction in the light energy passing through the peripheral region of the container bottom and the portion outside thereof in relation to the light energy passing through the central region of the container bottom [as shown in FIG. 15B]. Although such light energy reduction at the portion outside of the periphery of the container bottom is thought to be well indicative of the boundary of the container bottom and could be utilized to define the actual zone of inspection in correspondence with the container bottom, such energy reduction will not actually occur because of an internal reflection of light within the container 1, as seen in FIG. 16A. That is, the light after passing the container bottom and after being directed to the inner surface of the container sidewall is reflected thereat to be redirected toward the camera 40 as if it comes through the peripheral region or the sidewall outside of the container bottom. Since the camera 40 cannot discriminate such reflected light from the non-reflected light, the internally reflected light acts to add the light energy which the camera 40 recognizes as coming from other than the central region of the container bottom. With this result, the camera 40 provides the resulting light energy intensity distribution of FIG. 16B in which no clear optical characteristic is seen for indicative of the boundary of the container bottom, failing to define an inspection zone in exact coincidence with the actual container bottom.